I think the reason why they don't keep doing the Generations thing is because they realise the boost formula is a bit of a dead end. With that control scheme, all you can really do is go fast and sidestep obstacles and the platforming feels disjointed from the speed, as well as the controls being pretty clunky for the slower paced parts of the stages. Plus the boost being a 1-ht KO and the homing attack having incredible range means that enemies all die instantly and don't even get a chance to be threatening. That's what Lost World was about, trying (and failing but that's besides the point) to come up with a control scheme for Sonic that would better allow speed running and fast platforming as opposed to how Gens is a bit more on rails and has more flashy set pieces that don't do much more than look cool. The fish in Green Hill and the truck in modern City Escape for example. Of course, Sonic Team realised this when they first introduced this gameplay style in Sonic Unleashed. It's why the Werehog is a thing.
Having said that, I would still like more of Generations.